This invention is concerned with reinforcing preforms used in the production of metal matrix composites, and with the resulting reinforced metal. More particularly, this invention is concerned with preforms for use in making magnesium metal matrix composites.
It is well known that many metals, especially the so-called light metals, which generally includes aluminum, magnesium, and alloys of each of these metals, whilst having many of the advantages of a metal, such as the ability to be welded, and the particular advantage of low weight, also have the disadvantage that their strength properties are relatively low. In the past, this has limited the usefulness of these metals. One method that has been proposed to overcome this difficulty is to use the metals in the form of a composite, so that the composite more than compensates for the lack of mechanical, physical and other properties. Typical materials used as the reinforcement in such a composite include carbon, graphite, silicon carbide, titanium diboride, boron carbide and boron nitride. Techniques for making metal matrix composites from both aluminum and magnesium are well known, and have been described for example in U.S. Pat. No. 4,279,289; U.S. Pat. No. 4,715,442; U.S. Pat. No. 4,995,444 and U.S. Pat. No. 5,791,397.
These methods broadly involve a two step technique. In the first step, a preform is made from the reinforcing material, which is held together with a binder material. The commonly used binders are ceramic materials, of which silica is perhaps the commonest. The reinforcing material is generally invested with the binder system, and then fired both to burn off organic materials used to aid investment with the binder, and to convert the binder into silica. In the second step, the preform is invested with the molten metal to provide a composite material, typically by using the so-called squeeze casting technique, in which the molten metal is forced under high pressure into the preform contained in a mould.
Although these methods are more or less successful with aluminum, and aluminum alloys, they are less successful when used to fabricate composites in which the metal is magnesium, or a magnesium containing alloy. The difficulties arise from the fact that molten magnesium is a very reactive material, which has two consequences.
The first is that the use of silica as the preform binder becomes questionable, because the following reaction occurs:
2Mg+SiO2xe2x86x92Si+2MgO.
The silicon that is formed will dissolve into the magnesium to form a silicon-magnesium phase which generally will not have a deleterious effect on the properties of the magnesium metal, and thus of the resulting composite. The magnesium oxide is formed as a more or less crystalline solid, which does not dissolve in the magnesium metal. The presence of these crystals of magnesium oxide has been shown to have a deleterious effect on the overall properties of the composite when a sufficient amount is present. It can therefore be seen that this reaction places significant restrictions on the preform binder that can be used if the creation of magnesium oxide is to be avoided.
The second is that similar reactions will also happen with several of the other materials commonly used as either binders or reinforcement; all of the following reactions are known to occur:
3Mg+Al2O3xe2x86x922Al+3MgO
2Mg+TiO2xe2x86x92Ti+2MgO
Mg3(PO4)2+5Mgxe2x86x928MgO+2P
The aluminum and titanium alloy with the magnesium, and the phosphorus vaporizes. In the first two cases, although the aluminum and titanium have minimal, if any, effect on the properties of the resulting composite, this cannot be said for the magnesium oxide crystals that are produced in all three cases. The presence of brittle and weak magnesium oxide crystals makes the composite prone to crack formation. It can also be seen that these reactions place significant limitations on the materials which can be used to reinforce magnesium if the creation of magnesium oxide is to be avoided.
There is therefore a need for a binder system that can be used to create preforms for use in the manufacture of composites, in which the metal used is magnesium, or a magnesium alloy. Further, such a binder system should desirably be equally useful with both particulate and fibrous reinforcing materials, and should be potentially useful with other, less reactive, metals.
This invention seeks to provide a reinforcement preform for a magnesium metal matrix composite which involves the use of a preform binder system which is unreactive with molten magnesium, does not cause the creation of undesirable solid materials in the magnesium composite, and which can be used with the reinforcing materials commonly used with magnesium which do not involve the formation of magnesium oxide during the casting process. In addition, this invention also seeks to provide a reinforcement preform useful in composites in which the metal is not magnesium, such as aluminum and alunimum alloy composites.